that's a mis-statement. The general statement is that you will start to burn muscle AS WELL AS fat. However, the exact facts are as follows:
- it's bad to say muscle, as it should be noted as protein ... it's just that your muscle will be the main source of that protein
- yes, you will burn 'protein' at the higher HR's
- however, you will still be burning more fat than you do at the lower level because you are working a lot harder
It depends what your goals are, as far as which HR you exercise at. If you aren't into heavy weight training, or you have a lot of extra weight to lose, it's better to exercise at a higher HR.
If you trying to tone AND lose weight, then it's recommended to stay at a lower HR. For a guys, most of the time, we exercise in the lower HR because we are also trying to gain muscle.
.. but, on that note, you can help counter that issue. By eating a good source of protein before and after your workout, your body will burn that instead of your muscle (because that's what it's really looking for ... is a source of protein).